The present invention pertains to an apparatus for cutting strands such as mineral fiber like fiber glass, synthetic fibers like polyester or polyethylene and natural fibers like hemp and cotton, or for cutting ribbon like materials, and the method of using the apparatus, particularly to cut glass fiber continuously at high speed. The apparatus is called a multi-chopper because it has at least two chopper assemblies and is a vast improvement over the choppers used heretofore in that it greatly reduces downtime caused by chopper malfunctions or normal wear of the chopper assembly parts.
In processes of making chopped fiber of various kinds, a chopper receives continuously one or more strands made up of a plurality of fibers and chops the strand(s) into short lengths generally ranging from about 1/8th inch to 3 or more inches long. The strand(s) are often moving very fast through the chopper, typically at several thousand feet per minute. One example of such a process is the process of making chopped glass fiber as disclosed in U.S. Pat. Nos. 3,508,461, 3,771,701, 3,815,461, 3,869,268, 4,175,939, 4,249,441, 4,347,071, 4,373,650, 4,398,934, 4,411,180, 4,551,160, 4,576,621, and 4,840,755, which references are incorporated herein by reference. As time progresses blade edges and a backup roll on the chopper deteriorate to the point that the chopper is not cutting cleanly and "double cuts" or incomplete cuts are produced, i. e. one or more individual fibers are not cut leaving the chopped pieces linked together with one or more fibers. Since incomplete cuts are unacceptable, just prior to the elapsed time where incomplete cuts can no longer be avoided or where other factors such as vibration become serious, the chopper is shut down and the old backup roll and/or old blade roll are removed and replaced with rebuilt or new backup and/or blade rolls. This procedure requires at least 5-8 minutes and frequently more time, especially when it is necessary to replace another part on the chopper.
While the chopper is shut down for rebuild or repair, the fiber continues to issue from the fiberizing bushings on the line serviced by the chopper and must be sent to the basement as scrap, since it is impractical to stop the bushings from fiberizing. Also, the thermal balance on the fiberizing tips of the bushings are impacted negatively when the chopper is shut down because less external air is drawn into the tip area by the slow moving fibers as compared to when the chopper is pulling the fibers at thousands of feet per minute. This condition, it is believed, causes the thermal equilibrium of the bushing to be disturbed, and causes the glass exiting the tips to change temperature. If a chopper is down for more than a few minutes, this will cause the strand to break out (caused by fibers breaking) numerous times for many minutes after the chopper is back on line. This condition is sometimes referred to as "false starts" and this undesirable situation results in a significant reduction in fiberizing efficiency, i.e. a percentage obtained by dividing the weight of good fiber produced in a given period of time by the weight of molten glass that exited the bushings in the same period of time. Overheated bushings caused by chopper down times of 5 minutes or more can reduce fiber efficiency significantly for 10-20 minutes or more after the chopper is back on line. Also, while the bushings are "hanging", i.e. not fiberizing at high strand speed, such as when the strands are not being pulled at a speed of at least 1000 feet/minute by the chopper, etc., the melt rate of the bushings that are "hanging" drops significantly which changes the pull rate on the melter and upsets the equilibrium of the melter reducing glass quality and fiberizing efficiency.
The chopper has to be rebuilt on a regular basis and the time between rebuilds will vary depending on the diameter of the fiber being chopped, the type of chemical sizing on the surface of the fiber (most contain lubricants that complicate chopping), the condition of the chopper, the speed of chopping, and the quality of the lowest quality portion of the lowest quality blade edge or backup roll. Typical life times of blade rolls/backup rolls are in the range of 6-36 hours, depending on the type of fiber being chopped as explained above. A chopper typically services about 8-15 bushings, each putting out 100 or more pounds of fiber per hour, 24 hours per day and 7 days per week and a typical operation will have 8-20 choppers operating. Chopper down time typically costs at least ten dollars per minute per chopper. It can be readily seen that substantially reducing the down time of the choppers during rebuilds or repair will have a substantial positive financial impact on the operation.
This problem of substantial downtime of fiberization due to rebuilds of the choppers has persisted for many years in spite of the very substantial financial incentive to reduce or eliminate the problem.